It is often desirable to measure the shape or range of an object in digital data with an automatic system. The shape measuring of an object involves measuring the relative position of points which form the surface of the object. The art of measuring relief profiles with the optical method is a non-contact measurement and so does not affect the target. Parallel and high-resolution measurements are possible in this art.
Many kind of range sensors are often used for this purpose and the most common sensors use optical triangulation method to determine range. The triangulation method using a line-shaped laser sheet beam is practical due to the high-resolution and the high speed.
A conventional shape-measuring system using a line-shaped laser sheet beam is shown in FIG. 1. The system involves a line-shaped laser sheet beam on the surface of an object using a projector 1 consisting of a laser source and sheet beam generating optics and a camera 5 with an acute angle away from the line-shaped laser sheet beam to capture the deformed image of the line-shaped laser sheet beam. The method of measuring relief profiles involves the analysis of the above image with an image processing board 6 or a computer 7.
As shown in FIG. 2, when the surface of an object is not flat, the image of a line-shaped laser sheet beam is shifted in the direction vertical to the base line which is formed by the image of a line-shaped laser sheet beam projected on the reference flat area. Then, the vertical shift has the altitude-information of that point on the object surface. The shape-measuring, therefore, involves capturing the above image with a camera and then extracting the three-dimensional(3-D) information of the shape. The three-dimensional information over the whole surface of the object, however, is obtained by moving the target or scanning the line-shaped laser sheet beam.
Practically, the optical triangulation method with line-shaped laser sheet beam is most often utilized because of simplicity, high-resolution and high-speed. There are many prior patents which relate to the above shape-measuring apparatus, for example, U.S. Pat. No. 4,653,104 and U.S. Pat. No. 5,129,010.
The measurement resolution of this apparatus is limited by the finite number of CCD arrays the camera. To improve the resolution, the camera must be able to resolve small vertical shift of the line-shaped laser sheet beam. This generally requires high magnification in the direction of vertical shift. But, the high magnification with a conventional CCD camera lens cause to narrow the measuring range along the direction parallel to the length-wise direction of the image of line-shaped laser sheet beam.
As another method to improve the resolution, the image of an object can be digitally magnified only to the direction of vertical shift. The maximum image size along the direction of vertical shift is limited only by the maximum relief profile height. The measuring range along the direction parallel to the length-wise direction of the image of line-shaped laser sheet beam is not changed. In this case, the interpolation of magnified image is used to get the sub-pixel resolution of unmagnified image. However, this approach has a drawback in the long processing time for the interpolation of the image.
In the present invention, anisotropic magnification optics composed with one or more positive and one or more negative cylindrical lenses are used to obtain magnified images along the direction vertical to the length-wise direction of the image of a line-shaped laser sheet beam for higher measurement resolution, and non-magnified images along the direction parallel to the length-wise direction of the line-shaped image of laser sheet beam to measure the same range of area as the range without anisotropic magnification optics. Furthermore, in this invention, the anisotropic magnification can be varied by moving the positions of a positive and a negative cylindrical lenses and varying the interval between the cylindrical lenses while keeping the images in both width-wise and length-wise directions in focus.
There are similar prior inventions of optical triangulation system using an anamorphic optical system. For example, U.S. Pat. No. 5,621,814 by Honda suggests a method of inspecting height apparatus with a cylindrical lens and slit beams. But anamorphic optics with a cylindrical lens can image only one direction, therefore, anamorphic image of the line-shaped laser sheet beam by a cylindrical lens and a conventional camera lens has de-focused image blur through one-direction which degrade measurement resolution. U.S. Pat. No. 4,872,747 suggests optical systems with anamorphic prisms for use with laser triangulation optical sensor systems to improve measurement resolution.